Unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
The 2-branch downlink MIMO was introduced for HSDPA in 3rd Generation Partnership Project (3GPP) Standard Release 7. Then, 3GPP more focused on the carrier aggregation for the High Speed Downlink Packet Access (HSDPA) evolution. Dual-carrier HSDPA was introduced into 3GPP Standard Release 8. Further, the standardization of 4-carrier HSDPA and dual-carrier High Speed Uplink Packet Access (HSUPA) was almost finished. In 3GPP Standard Release 11, 8-carrier HSDPA was accepted as a work item in 3GPP.
Compared to the carrier aggregation, multiple antenna technology more helps to improve the frequency utilization efficiency as well as the coverage. In good channel conditions, the data transmission in a high rank channel increases the peak data rate. In bad channel conditions, the beamforming gain in a low rank channel improves the coverage. Presently, only 2-branch Multiple Input Multiple Output (MIMO) was specified for HSDPA. Compared to the up to 8-layer data transmission in Long-Term Evolution (LTE) downlink, the number of branch in MIMO transmission of HSDPA can be further increased. In 3GPP RAN1-65 meeting, it was proposed to introduce the 4-branch MIMO for HSDPA (cf. Reference [1]).
For downlink, there are control channels and data channels. For HSDPA, the downlink physical channel that carries data is High Speed Physical Downlink Shared Channel (HS-PDSCH). The spreading factor is 16, and any other spreading factor is not allowed. There are mainly two types of pilot designs being discussed in 3GPP, common pilots only, and common pilots plus shared demodulation pilots. These two types of pilot options are described in detail below.                OPTION 1: COMMON PILOTS ONLY         FIG. 1 exemplifies the structure of 4-branch MIMO with common pilots only. There are one Primary Common Pilot Channel (CPICH) (P-CPICH) plus x (0-3) Secondary CPICHs (S-CPICH), which are used for data demodulations over High Speed Downlink Shared Channel (HS-DSCH) and High Speed Shared Control Channel (HS-SCCH), Channel Quality Indicator (CQI) estimation, and Pre-coding Matrix Indicator (PMI) and rank selection. Dependent on the number of S-CPICHs, there can be multiple orthogonal patterns for the P-CPICH and/or S-CPICH. As shown in FIG. 1, there are in total four common pilots, two P-CPICH patterns and two S-CPICH patterns. The two P-CPICH patterns are orthogonal to each other, and the two S-CPICH patterns are orthogonal to each other.        Non-MIMO User Equipments (UEs) can use the default P-CPICH pattern, or one S-CPICH pattern configured for data demodulations and CQI estimation. 2-branch MIMO UEs can be configured to use the two orthogonal P-CPICH patterns, or one P-CPICH pattern plus one S-CPICH pattern. 4-branch MIMO UEs (i.e., the UE configured in 4-branch MIMO modes) will use all these four common pilots.        OPTION 2: COMMON PILOTS PLUS SHARED DEMODULATION PILOTS (SDPs)        FIG. 2 exemplifies the structure of 4-branch MIMO with common pilots plus SDPs. There are still four common pilots, which can be any of the cases mentioned in Option 1. The UEs in legacy modes (i.e., non-MIMO UEs and 2-branch MIMO UEs) may operate in the same way as mentioned in Option 1. The 4-branch MIMO UEs uses common pilots for CQI, PMI and rank estimation. The channel estimation for the data demodulations of the 4-branch MIMO UEs relies on the 4 SDPs.        
For any option, the common pilots are required to get the raw channel for pre-coding matrix, rank estimation and CQI measurement.
Compared to 2-branch MIMO, there are two additional common pilots (i.e., the 3-rd and the 4-th common pilots) for spatial channel estimation, which mean additional power consumption and interference. The 4-branch MIMO capable UEs can support up to 4 antenna ports. In case of good channel conditions, the 4-branch MIMO capable UEs can be served with up to 4 data-stream transmission. In case of bad channel conditions, the 4-branch MIMO capable UEs can benefit from beamforming transmission with a low rank channel. While for legacy UEs (i.e., non-MIMO UEs and 2-branch MIMO UEs), for instance, the non-MIMO UEs can only support single antenna port and the 2-branch MIMO UEs can support up to 2 antenna ports. For non-MIMO UEs, only one CPICH is used for channel estimation and CQI measurement, and the other three common pilots are useless but consume the transmit power and generate interference. For 2-branch MIMO UEs, the last two CPICHs are not used. Such impact can be severer if the additional common pilots are transmitted with higher powers.
According to the current studies, there is around 30% loss for 2-branch MIMO or non-MIMO UE if the 3-rd and 4-th common pilots are set to −13 dB relative to the maximum downlink power.